[POWERPC] Fix make rules for treeImage.initrd

At present attempting to build treeImage.initrd.* boot images will
fail, because make will select the treeImage.% rule which also matches
instead of the correct and more specific treeImage.initrd.% rule.
This patch corrects the problem by listing the more specific rule
first.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>